1. Field of the Invention
The present invention relates to an apparatus for forming and providing color pictures in color copying machines, color printers, facsimile and the like, and more particularly to a digital image processing apparatus in which a color image signal is digitally processed for color conversion and adjustment of the image to be reproduced.
2. Prior Art
Conventionally, a a color picture forming apparatus has been provided in the form of a large sized printing apparatus in publishing and printing industries. However, an apparatus having sufficient functions for general use is expensive in cost, and therefore only a simple color printer suitable for hobby use has been provided in the market.
Recently, more attention has been given to a color printing in the industry. For example, the color printing is applied to news paper. With such an increasing demand for color printing, the color copying machines have become more commercially acceptable in terms of cost.
A color picture forming apparatus such as a copying machine is equipped with a developing machine for toners consisting of Y (Yellow), M (magenta), C (cyan), and K (black) for reproducing a full-color image, in which the respective color toners are transferred and printed over the preceding colors to reproduce a color picture. In other words, four successive copying processes have to be carried out for obtaining a full-color picture. Thus, a copy operation for copying a color picture of a color original placed on a platen is carried out as follows. The color picture of the original is first optically read out to be optically separated into the respective colors, and then the signals so read out are converted into a density signal for each toner. On the basis of the density signal, a modulation signal for modulating a laser light to which a photosensitive material is exposed is produced, to thereby perform the copy at an image output terminal. Particularly, reproducing a color picture includes reproduction of pictures having intermediate gradation such as photographs and paintings and reproduction of pictures including lines and characters, and therefore high level of signal processing is required for reproducing the colors and gradation according to both reproductions.
Generally, a large capacity of memory is required if four image signals obtained by one optical scanning operation of the original document are simultaneously stored for the four copying processes. Accordingly, the scanning operation and the subsequent processing of the image signals thus obtained are carried out every copy process and successively repeated at four times as a whole. In this method, the image signals are obtained as gradation signals of colors B (blue), G (green), and R (red) into which the picture is optically separated, and are converted into toner signals for those colors. As described above, the image signal is required to be subjected to various processings. Accordingly, the signal is converted into a digital signal at a first stage, and then to a binary signal of ON and OFF, at a final stage. The binary signal is used to control a laser beam to reproduce a picture having intermediate gradation through development of dot gradation.
As described above, a conventional full-color copying machine carries out four copying processes to superpose the respective color pictures on one another, and more attention is given to improve reproducibility of color, gradation, and fineness of the picture.
For example, color-separation signals of B, G and R which have been obtained by optically reading the original document are not matched up color balance due to the difference between characteristics of the respective sensors and the image thereof suffers blur. Thus, it is the most important problem how the blur is eliminated from such signals and color-balanced toner signals are obtained. Further, there occurs a problem that addition of K (black) causes muddiness when K is added to Y, M, and C. Still further, how smooth tone and emphasis of an edge are adjusted in a picture of letters and a picture having intermediate gradation such as a photograph or a painting is another problem.
While the color picture forming apparatus is expected to be capable of performing a wide variety of editing such as color conversion, mono-color, full-color, a line drawing for coloring, another problem occurs what actual editing functions are to be provided and how they are to be carried out.